1. Field
The exemplary and non-limiting embodiments of this invention relate generally to an imaging spectrometer.
2. Description of the Related Art
A spectrometer is an optical device which receives optical radiation and separates light by wavelengths to produce a spectrum. The spectrum, which may also be called a spectral density, is a distribution of intensity of the optical radiation input to the spectrometer as a function of a wavelength. A detecting element transforms the spectrum into an electrical form after which a signal processor may be used to analyze the spectrum by, for example, quantifying the amount of each wavelength component that is present in the input optical radiation.
An imaging spectrometer is a device which allows simultaneous spatial and spectral information to be collected from a sample. The imaging spectrometer provides a distribution of intensity of the optical radiation as a function of both a wavelength and a location.
There are several kinds of optical configurations for the spectrometers, depending on the nature of operation principle. Prism(s), concave or plane reflective gratings or transmission gratings can be used as the dispersive component. Both reflective mirrors and lenses may be used for collimating and focusing and they can modify the wavefronts incident on and diffracted by the grating. Some commonly known layouts are Czerny-Turner-, Ebert- and Off-ner-designs.
There are problems with older designs, such as Czerny-Turner- and Ebert-types. The designs have a limited flat field image, and a strong aberration due to mirrors, which are on both sides of the dispersive component. Additionally, the reflective surfaces are at an angle with respect to the optical axis i.e. they are optical subsystems operating in off-axis.
A dominant off-axis aberration associated to an off-axis reflective optical component is usually coma, but also astigmatism appears. The main image surface deformation is a field curvature. Additionally, smile and Key-stone become a problem. Hence, a point becomes a large spot and image is distorted on the detecting element and this is a reason why they cannot be used with a two-dimensional detecting element in a high resolution spectroscopy. Offner-design is only a slightly better because of a curved surface grating which is an attempt to compensate the aberrations caused by the off-axis mirrors. However, Offner-design still has too strong aberrations for spectroscopy with a good resolution.
That detecting is performed at the same side of the spectrometer as the input of the optical radiation is also common to Czerny-Turner-, Ebert- and Offner-designs. Such a configuration causes the detector, such as a camera, to physically hinder the optical input to the spectrometer. An additional mirror may be used to deflect the optical output for obtaining space for the input but this only increases the already disadvantageous complicatedness and cost of the spectrometer.
In some spectrometers, at least one refractive optical component is used to collimate the optical radiation before the dispersive component instead of a mirror for an optical on-axis operation to avoid off-axis aberrations. However, the at least one refractive optical component causes a chromatic aberration to the optical radiation to be dispersed which naturally deteriorates the accuracy of the spectrometer substantially. Additionally, a zero-deviation dispersive component itself causes smile and Keystone which are unsymmetrical relative to the optical axis, which deteriorates the accuracy of the spectrometer, too. Hence, there is a need for a good imaging spectrometer.